1. Field of the Invention
The present invention relates to a process for producing 2,2',5,5',6,6'-hexafluorobiphenyl-3,3',4,4'-tetracarboxylic acid precursors which are useful as intermediate materials for highly functional fluororesins with excellent heat resistance, light resistance, low moisture absorption and low permittivity.
2. Description of the Related Art
Recent years have seen an increased demand for resins with high functionality. Functional materials having high heat resistance, low moisture absorption and low permittivity have been particularly desired in the electronics industry.
Fluorinated polyimides are examples of materials which can meet this demand, and fluorinated tetracarboxylic dianhydrides are known as aromatic tetracarboxylic acid components thereof.
Among these there is known 2,2',5,5',6,6'-hexafluorobiphenyl-3,3',4,4'-tetracarboxylic dianhydride, represented by the following formula (4). ##STR1##
This 2,2',5,5',6,6'-hexafluorobiphenyl-3,3',4,4'-tetracarboxylic dianhydride is usually produced by anhydrating 2,2',5,5',6,6'-hexafluorobiphenyl-3,3',4,4'-tetracarboxylic acid.
Only two types of processes are known for their synthesis. One is the process described in U.S. Pat. No. 3,440,277 (1969). This patent specification gives examples of producing 2,2',5,5',6,6'-hexafluorobiphenyl-3,3',4,4'-tetracarboxylic acid, 2,2',5,5',6,6'-hexafluorobiphenyl-3,3',4,4'-tetracarboxylic acid tetramethyl ester and 2,2',5,5',6,6'-hexafluorobiphenyl-3,3',4,4'-tetracarboxylic dianhydride. However, the process described in this patent specification is a synthesis process requiring multiple stages employing a polyfluoroaromatic lithium compound during the synthesis, and because this compound necessitates special care for handling and increases costs, this process has been disadvantageous for industrial production.
The other process is described in Japanese Unexamined Patent Publication No. 3-101673. This process uses 4-bromo-3,5,6-trifluorophthalonitrile and copper powder to synthesize 3,3',5,5',6,6'-hexafluoro-4,4'-biphthalonitrile by the Ullmann reaction. The same publication also describes hydrolysis and anhydration of the 3,3',5,5',6,6'-hexafluoro-4,4'-biphthalonitrile to obtain 3,3',5,5',6,6'-hexafluoro-4,4'-biphthalic dianhydride in an approximately quantitative manner.
This process requires a large excess of copper in the production step for the 3,3',5,5',6,6'-hexafluoro-4,4'-biphthalonitrile (2,2',5,5',6,6'-hexafluorobiphenyl-3,3',4,4'-tetracarbonitrile) and, with a yield as low as about 18%, it is not satisfactory as an industrial production process.
In summary, to date there has been no industrially practical production process for high-yield production of 2,2',5,5',6,6'-hexafluorobiphenyl-3,3',4,4'-tetracarboxylic acid precursors such as 2,2',5,5',6,6'-hexafluorobiphenyl-3,3',4,4'-tetracarbonitrile and 2,2',5,5',6,6'-hexafluorobiphenyl-3,3',4,4'-tetracarboxylic tetraalkyl esters, which are useful as synthetic intermediates for 2,2',5,5',6,6'-hexafluorobiphenyl-3,3',4,4'-tetracarboxylic dianhydride.